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Bacterial surface appendages strongly impact nanomechanical and electrokinetic properties of Escherichia coli cells subjected to osmotic stress.

Francius G, Polyakov P, Merlin J, Abe Y, Ghigo JM, Merlin C, Beloin C, Duval JF - PLoS ONE (2011)

Bottom Line: Additionally, for a given surface appendage, the magnitude of the nanomechanical parameters decreases significantly when increasing bulk salt concentration.This effect is ascribed to a bacterial exoosmotic water loss resulting in a combined contraction of bacterial cytoplasm together with an electrostatically-driven shrinkage of the surface appendages.Altogether, AFM and electrokinetic results clearly demonstrate the intimate relationship between structure/flexibility and charge of bacterial envelope and propensity of bacterium and surface appendages to contract under hypertonic conditions.

View Article: PubMed Central - PubMed

Affiliation: Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, Nancy Université, CNRS UMR7564, Villers-lès-Nancy, France. gregory.francius@lcpme.cnrs-nancy.fr

ABSTRACT
The physicochemical properties and dynamics of bacterial envelope, play a major role in bacterial activity. In this study, the morphological, nanomechanical and electrohydrodynamic properties of Escherichia coli K-12 mutant cells were thoroughly investigated as a function of bulk medium ionic strength using atomic force microscopy (AFM) and electrokinetics (electrophoresis). Bacteria were differing according to genetic alterations controlling the production of different surface appendages (short and rigid Ag43 adhesins, longer and more flexible type 1 fimbriae and F pilus). From the analysis of the spatially resolved force curves, it is shown that cells elasticity and turgor pressure are not only depending on bulk salt concentration but also on the presence/absence and nature of surface appendage. In 1 mM KNO(3), cells without appendages or cells surrounded by Ag43 exhibit large Young moduli and turgor pressures (∼700-900 kPa and ∼100-300 kPa respectively). Under similar ionic strength condition, a dramatic ∼50% to ∼70% decrease of these nanomechanical parameters was evidenced for cells with appendages. Qualitatively, such dependence of nanomechanical behavior on surface organization remains when increasing medium salt content to 100 mM, even though, quantitatively, differences are marked to a much smaller extent. Additionally, for a given surface appendage, the magnitude of the nanomechanical parameters decreases significantly when increasing bulk salt concentration. This effect is ascribed to a bacterial exoosmotic water loss resulting in a combined contraction of bacterial cytoplasm together with an electrostatically-driven shrinkage of the surface appendages. The former process is demonstrated upon AFM analysis, while the latter, inaccessible upon AFM imaging, is inferred from electrophoretic data interpreted according to advanced soft particle electrokinetic theory. Altogether, AFM and electrokinetic results clearly demonstrate the intimate relationship between structure/flexibility and charge of bacterial envelope and propensity of bacterium and surface appendages to contract under hypertonic conditions.

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Nanomechanical properties of bacteria investigated in 1 mM KNO3.Deflection images (first column), elasticity maps (scale 0–20 MPa) (second column) and Young modulus distributions (third column) obtained in 1 mM KNO3 solution for a) E2152 b) E2146 c) E2302 d) E2498. Young modulus distributions were calculated within the spatial range marked by the white window positioned in the corresponding elasticity map (n = 256 force curves). Elastic (Young) moduli and bacterial spring constants were extracted from typical force-indentation curves shown in insets: open symbols are raw AFM data, solid lines stand for theoretical fits according to eq 5 (blue color, Hertzian non-linear regime) and straight lines represent fits on the basis of eq 4 (red color, linear or compliance regime associated to Turgor pressure contribution).
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pone-0020066-g004: Nanomechanical properties of bacteria investigated in 1 mM KNO3.Deflection images (first column), elasticity maps (scale 0–20 MPa) (second column) and Young modulus distributions (third column) obtained in 1 mM KNO3 solution for a) E2152 b) E2146 c) E2302 d) E2498. Young modulus distributions were calculated within the spatial range marked by the white window positioned in the corresponding elasticity map (n = 256 force curves). Elastic (Young) moduli and bacterial spring constants were extracted from typical force-indentation curves shown in insets: open symbols are raw AFM data, solid lines stand for theoretical fits according to eq 5 (blue color, Hertzian non-linear regime) and straight lines represent fits on the basis of eq 4 (red color, linear or compliance regime associated to Turgor pressure contribution).

Mentions: To determine the nanomechanical properties of the four bacterial strains of interest in this study, force-distance curves were recorded, converted into force-indentation curves, and subsequently analyzed according to eqs 4 and 5 for determining the bacterial spring constant, related to the inner Turgor pressure of the cell, and the Young modulus that reflects cell surface elasticity/rigidity. Also, use of 3D force volume mode rendered possible the generation of spatial distribution of Young Modulus (elasticity map) at the bacterial surface. Fig 4 shows these elasticity maps for the four cells (a, b, c and d) in 1 mM KNO3 solution, together with the dispersion in Young modulus values as determined within the spatial region indicated by the white windows on the elasticity maps. Also, for the sake of illustration, typical force-indentation curves are given for each strain and were found to be in agreement with the force-indentation relationship expected from Sneddon model (eq 5), thereby rendering possible determination of Young moduli. Consistent with the work of Gaboriaud et al. [65], the force curves recorded on cell surfaces exhibit a non-linear regime at low loading forces (eq 5) followed by a linear regime for higher forces (eq 4). Bacterial spring constant values, kcell, were determined from the slopes of the linear portions of the force-versus-piezo displacement curves as described in Material & Methods section.


Bacterial surface appendages strongly impact nanomechanical and electrokinetic properties of Escherichia coli cells subjected to osmotic stress.

Francius G, Polyakov P, Merlin J, Abe Y, Ghigo JM, Merlin C, Beloin C, Duval JF - PLoS ONE (2011)

Nanomechanical properties of bacteria investigated in 1 mM KNO3.Deflection images (first column), elasticity maps (scale 0–20 MPa) (second column) and Young modulus distributions (third column) obtained in 1 mM KNO3 solution for a) E2152 b) E2146 c) E2302 d) E2498. Young modulus distributions were calculated within the spatial range marked by the white window positioned in the corresponding elasticity map (n = 256 force curves). Elastic (Young) moduli and bacterial spring constants were extracted from typical force-indentation curves shown in insets: open symbols are raw AFM data, solid lines stand for theoretical fits according to eq 5 (blue color, Hertzian non-linear regime) and straight lines represent fits on the basis of eq 4 (red color, linear or compliance regime associated to Turgor pressure contribution).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3105017&req=5

pone-0020066-g004: Nanomechanical properties of bacteria investigated in 1 mM KNO3.Deflection images (first column), elasticity maps (scale 0–20 MPa) (second column) and Young modulus distributions (third column) obtained in 1 mM KNO3 solution for a) E2152 b) E2146 c) E2302 d) E2498. Young modulus distributions were calculated within the spatial range marked by the white window positioned in the corresponding elasticity map (n = 256 force curves). Elastic (Young) moduli and bacterial spring constants were extracted from typical force-indentation curves shown in insets: open symbols are raw AFM data, solid lines stand for theoretical fits according to eq 5 (blue color, Hertzian non-linear regime) and straight lines represent fits on the basis of eq 4 (red color, linear or compliance regime associated to Turgor pressure contribution).
Mentions: To determine the nanomechanical properties of the four bacterial strains of interest in this study, force-distance curves were recorded, converted into force-indentation curves, and subsequently analyzed according to eqs 4 and 5 for determining the bacterial spring constant, related to the inner Turgor pressure of the cell, and the Young modulus that reflects cell surface elasticity/rigidity. Also, use of 3D force volume mode rendered possible the generation of spatial distribution of Young Modulus (elasticity map) at the bacterial surface. Fig 4 shows these elasticity maps for the four cells (a, b, c and d) in 1 mM KNO3 solution, together with the dispersion in Young modulus values as determined within the spatial region indicated by the white windows on the elasticity maps. Also, for the sake of illustration, typical force-indentation curves are given for each strain and were found to be in agreement with the force-indentation relationship expected from Sneddon model (eq 5), thereby rendering possible determination of Young moduli. Consistent with the work of Gaboriaud et al. [65], the force curves recorded on cell surfaces exhibit a non-linear regime at low loading forces (eq 5) followed by a linear regime for higher forces (eq 4). Bacterial spring constant values, kcell, were determined from the slopes of the linear portions of the force-versus-piezo displacement curves as described in Material & Methods section.

Bottom Line: Additionally, for a given surface appendage, the magnitude of the nanomechanical parameters decreases significantly when increasing bulk salt concentration.This effect is ascribed to a bacterial exoosmotic water loss resulting in a combined contraction of bacterial cytoplasm together with an electrostatically-driven shrinkage of the surface appendages.Altogether, AFM and electrokinetic results clearly demonstrate the intimate relationship between structure/flexibility and charge of bacterial envelope and propensity of bacterium and surface appendages to contract under hypertonic conditions.

View Article: PubMed Central - PubMed

Affiliation: Laboratoire de Chimie Physique et Microbiologie pour l'Environnement, Nancy Université, CNRS UMR7564, Villers-lès-Nancy, France. gregory.francius@lcpme.cnrs-nancy.fr

ABSTRACT
The physicochemical properties and dynamics of bacterial envelope, play a major role in bacterial activity. In this study, the morphological, nanomechanical and electrohydrodynamic properties of Escherichia coli K-12 mutant cells were thoroughly investigated as a function of bulk medium ionic strength using atomic force microscopy (AFM) and electrokinetics (electrophoresis). Bacteria were differing according to genetic alterations controlling the production of different surface appendages (short and rigid Ag43 adhesins, longer and more flexible type 1 fimbriae and F pilus). From the analysis of the spatially resolved force curves, it is shown that cells elasticity and turgor pressure are not only depending on bulk salt concentration but also on the presence/absence and nature of surface appendage. In 1 mM KNO(3), cells without appendages or cells surrounded by Ag43 exhibit large Young moduli and turgor pressures (∼700-900 kPa and ∼100-300 kPa respectively). Under similar ionic strength condition, a dramatic ∼50% to ∼70% decrease of these nanomechanical parameters was evidenced for cells with appendages. Qualitatively, such dependence of nanomechanical behavior on surface organization remains when increasing medium salt content to 100 mM, even though, quantitatively, differences are marked to a much smaller extent. Additionally, for a given surface appendage, the magnitude of the nanomechanical parameters decreases significantly when increasing bulk salt concentration. This effect is ascribed to a bacterial exoosmotic water loss resulting in a combined contraction of bacterial cytoplasm together with an electrostatically-driven shrinkage of the surface appendages. The former process is demonstrated upon AFM analysis, while the latter, inaccessible upon AFM imaging, is inferred from electrophoretic data interpreted according to advanced soft particle electrokinetic theory. Altogether, AFM and electrokinetic results clearly demonstrate the intimate relationship between structure/flexibility and charge of bacterial envelope and propensity of bacterium and surface appendages to contract under hypertonic conditions.

Show MeSH
Related in: MedlinePlus